The PTC devices of this invention are useful, for example, in the form of self-regulating heater cable for process control, for example, viscosity maintenance in chemical plants, oil refineries, etc. They are also useful for freeze protection of pipes, valves, vessels and the like in such industrial applications as food processing plants, power stations, refineries, chemical plants, off-shore platforms, steel mills and commercial buildings.
PTC (positive temperature coefficient) compositions and devices which exploit them are well known in the art. When a constant voltage is applied across the composition, the current and the resistance stay approximately constant so long as the PTC composition is at low temperature. When the PTC composition heats up, it reaches a "switching" temperature or temperature range where its resistance increases dramatically (a factor of six or more), and since the voltage is constant, current decreases. Accordingly, PTC devices act essentially as temperature self-regulating devices.
Self-regulating heater articles and/or cable have been available for many years. Typically, the cable article comprises a pair of spaced elongated electrodes or wires that are embedded in a semiconductive core material that exhibits a PTC characteristic. The core material composition comprises a crystalline polymer in which is dispersed a carbon black filler with a typical loading in the 12-20% range by weight of the total composition,.
The temperature up to which the heater article can be used (upper use temperature) is determined primarily by the type of polymer used in the PTC composition. In low temperature applications, the PTC composition polymer is typically polyethylene which has a use temperature up to 85.degree. C. The industry has been capable of producing self-regulating heater articles with polyethylene based PTC compositions with a reasonable production yield. The carbon black filler in some cases is highly conductive, such as, Vulcan XC72 (U.S. Pat. No. 3,861,029) in other cases is highly resistive, such as Mogul L or Raven 1255 (U.S. Pat. Nos. 4,277,673, 4,327,480 and 4,367,168) and in still other cases is a blend of blacks (U.S. Pat. Nos. 4,277,673, 4,327,480 4,367,168 and the Narkis article at pages 1163-115,volume 22, Journal of Applied Polymer Science, 1978). The carbon blacks, Vulcan XC72 and Mogul L, are available from Cabot Corporation, Waltham, Mass. and Raven 1255 is available from Cities Service Co. Typical production processes are described in U.S. Pat. No. 4,327,480 and in U.S. Pat. No. 4,866,253.
The process described in Patent No. 4,327,480 is particularly advantageous in that it is characterized by short anneal times that permit a continuous feed of the cable extrudate through an anneal oven. That is, annealing is performed while the cable extrudate is in transit. A significant advantage of this process is that the power rating of the cable product can be controlled as a function of (1) the anneal time or speed of the cable extrudate through the anneal oven, and (2) the temperature of the oven.
For self-regulating heater articles intended for high temperature applications, it has been customary for 10 years or so to use polymers with much higher melting points, such as fluoropolymers (for example, ethylene-tetrafluoroethylene copolymer, with a melting point of about 270.degree. C., U.S. Pat. No. 4,591,700). Self-regulating heater articles using fluoropolymers have been plagued with production yield problems that result in higher production costs and market prices. Typically, these problems involve nonuniformity of article resistance made from (1) a single batch of core material and (2) different batches of the same mix. This has caused resort to screening tests in which a production run is tested at various points along the length for hot and cold spots. The screening test involves measuring temperatures at different locations along the cable length with voltage applied at either or both ends of the cable. The measured temperatures are correlated with power output as measured at each end of the cable. Cold spots of the cable are cut out and scrapped. This obviously increases the cost of production as well as market price of finished heater articles.
One prior art self-regulating heater cable intended for high temperature applications is similar to the cable described in U.S. Pat. No. 4,459,473 which employs two wires which are held in spaced apart relation by a spacer and a PTC heating strip helically wound about the wires. This product has been reported as having experienced arcing at the strip/wire contact points, particularly at higher voltages. The product is also expensive to make because of the operations required to implement the spaced wires, the PTC strip and the winding of the strip about the spaced wires.